1. Field of the Invention
The invention relates to a high-convection gas jet nozzle section for sheet-like material guided over rollers, in particular for the thermal tempering of thin flat glass sheets, comprising a lower nozzle field having nozzle ribs which are arranged centrally and parallel to each other or parallel to the rollers between the rollers and are provided with nozzle apertures, and an upper nozzle field of which the nozzle ribs provided with nozzle apertures are arranged symmetrically to the vertical axis of the opposite lower nozzle ribs.
2. Description of the Prior Art
Using such high-convection gas jet nozzle sections or lines the sheet-like material guided over the rollers, for example a metal plate, a strip of metal, plastic, fibres, textiles or sections of such materials, can be heated or cooled.
Particularly advantageous is the use of such a high-convection gas jet nozzle section as cooling section for thermal toughening or tempering of thin flat glass sheets led over rollers because in this case on the one hand particularly high heat transfers must be achieved and on the other the glass sheets must not be lifted from the roller path, i.e. in spite of the rollers at the lower side of the glass influencing the flow at the glass sheet almost equalizing pressure distributions must arise at the glass upper and lower side even with very intense blasting of the glass sheet.
The heat transfer coefficient to be obtained in a cooling section for thermal tempering of glass is approximately inversely proportional to the glass thickness. Usually, the heat transmission is ensured by blowing air onto the glass sheet heated to its softening temperature, the air coming from a nozzle system which is supplied by a fan. With this type of heat transfer the heat transfer coefficient increases only with about the 0.7th power of the nozzle exit velocity. The flow power necessary for this purpose however increases with the third power of the nozzle exit velocity. Thus, if for example in a cooling section glass having a thickness of 3 mm instead of a thickness of 5 mm is tempered, then for this purpose the nozzle exit velocity would have to be increased by about 2.1 times and the flow power by 9 times in order to achieve the same heat transfer.
In the present state of the art, such an increase in the power is the usual method of extending the range of use of a cooling section for tempering glass for small glass thicknesses. Thus, in industrially operated cooling sections for tempering planar 3 mm thick glass sheets nozzle pressures of 20000 to 22000 pa are used.
However, due to these high pressures, for thin glass sheets whose weight decreases linearly with the thickness and whose stiffness decreases with the third power of the thickness additional difficulties are encountered in the guiding of the glass sheet over rollers. For to achieve adequate planarity, it would be necessary for both the high heat transfer and the pressure forces to be of equal magnitude at the upper and lower sides of the glass sheets led over rollers. However, at the lower side the pressure action and heat transmission is impaired by the rollers. Nevertheless, in accordance with the prior art at the upper and lower sides the same nozzle field is used for blasting the glass sheets, the necessary difference in the heat transfer being compensated by different operating pressures for the upper and lower nozzle fields. Since the rollers dam up the flow at the lower side, the pressure at the lower side must be increased with respect to the pressure at the sheet upper side to compensate the heat transfer difference caused thereby. With thick sheets this pressure difference can be compensated by the comparatively high weight of the sheet or pane so that in spite of a pressure difference adequate guiding of the sheet by the rollers is provided. However, with thin sheets the necessary substantially higher nozzle pressure at the lower side and the resulting pressure differential force directed from below upwardly lead to lifting of the sheets from the roller group and thus to considerable production disturbances, if indeed tempering of thin glass is possible at all with such apparatus.
As a remedy for this problem, between the upper nozzle ribs dummy rollers, for example cylindrical tubes, are located which generate a dam effect for the tempering air applied to the sheet similar to that caused by the rollers beneath the sheet. Since however these dummy rollers, in contrast to the rollers themselves, do not contact the surface of the sheet but must be held at a certain minimum distance above the glass surface, the flow fields and thus the pressure fields, and consequently finally the heat transfer differ at the upper and lower side of the glass disc to such a considerable extent that with such apparatuses a sheet safety glass with thicknesses below 3.2 mm can be made only with considerable restrictions as regards the demands on optical quality and the fracture pattern. German patent 3,150,859 (corresponding to U.S. Pat. No. 4,586,946) discloses a nozzle system having nozzle ribs which are adapted to the form of an arched glass disc and from which gas jets are directed both vertically and also inclined onto the arched glass disc. Above and below the glass sheets or discs the same nozzle ribs are employed, the influence of the rollers on the flow behaviour described above being compensated by throttle rods arranged between the upper nozzles. However, as practical experience has shown with such a constructional form relatively great differences still occur in the heat transfer at the upper and lower sides of the glass sheet.
Furthermore, German patent 2,256,087 discloses an apparatus for drying a web of material having a nozzle system generating jets which without exception impinge inclined on the material web; the blast direction of each nozzle hole row, starting from the nozzle box centre and with respect to the material web, is at a progressively smaller blast angle from hole row to hole row; in addition, the clear distance from the material web at each nozzle hole row becomes stepwise greater than at the preceding nozzle hole row progressively starting from the nozzle box centre towards the two nozzle box edges. This nozzle developed for the drying art is specifically designed for avoiding thin relatively sensitive material webs to be dried lifting off the support on which they are guided. For this reason, a jet inclination angle of 15.degree. to 45.degree. can also be accepted. However, with such large jet inclination angles it is not possible to obtain the extremely high heat transfer required for example for tempering thin safety glass sheets or panes.